Under physiological conditions, the transport mechanisms that regulate cell volume in duck erythrocytes require C1. C1 seems to be co-transported with K or Na plus K during these transport processes. This dependence on C1 is a linear function of the intracellular and/or extracellular chloride concentration. By inhibiting the chloride fluxes associated with the anion exchange mechanism, one can unmask these cation-coupled chloride fluxes and observe them directly. The evidence indicates that this C1 movement occurs through membrane pathways not related to the anion exchanger.